Electropolishing apparatus and method for implantable medical devices

ABSTRACT

An electropolishing device for polishing a cylindrical medical device such as a stent includes anodes in the form of rollers arranged to contact an exterior surface of the device. A drive mechanism rotates the anodes periodically during the electropolishing process to change a contact point between the anodes and the medical device. A tilting mechanism can also be used for periodically tilting the anodes during the electropolishing process to allow bubbles to escape from one end of the stent.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/708,010, filed Aug. 12, 2005, the entire contents of whichare incorporated here by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electropolishing devices and methods,and more particularly to electropolishing devices for polishing stentsand other cylindrical medical devices.

2. Summary of the Related Art

In the past, permanent or biodegradable devices have been developed forimplantation within a body lumen or blood vessel to maintain patency ofthe lumen. Once expanded within the blood vessel, these devices, calledstents, become encapsulated within the body tissue and provide apermanent or biodegradable support for the vessel wall.

Known stent designs include thin-walled metal cylinders with axial slotsformed around the circumference (U.S. Pat. Nos. 4,733,665; 4,739,762;and 4,776,337). Known construction materials for use in stents includepolymers, organic fabrics and biocompatible metals, such as, stainlesssteel, gold, silver, tantalum, titanium, cobalt chromium alloys, andshape memory alloys, such as Nitinol.

Stents are often formed by cutting a pattern into a tube, such as astainless steel or cobalt chromium alloy tubes with a laser. However,laser cutting and other metal forming operations tend to leave burrs andsharp edges which are undesirable on implantable devices. The stentswill be in contact with tissue and burrs and sharp edges can causetissue irritation. Tissue irritation caused by burrs and rough surfaceson stents can be one of the causes of restenosis or the re-occlusion ofa blood vessel after balloon angioplasty and stenting. In addition, whensharp edges, burrs, or other marks are on the inside of stents they cancause disturbance in the blood flow through a blood vessel which cancause the blood to clot. Electropolishing is a process used forproviding a smooth surface on metal parts including implantable metallicdevices, such as stents.

Electropolishing involves immersing a metallic object into anelectrolyte solution in the presence of an electric current which causesprotrusions on the metal object to be dissolved. The metallic object,such as the stent, is positioned in contact with an anode or positiveelectrode, while a cathode or negative electrode is positioned in theelectrolyte solution nearby. As the electric current passes through themetallic object and electrolyte solution, from the anode to the cathode,metal is removed from the object. However, the metal is removed fasterat the locations of protruding portions of the metal due to aconcentration of the current at these locations. The dissolving of theprotrusions faster than the remaining metal surfaces results in a verysmooth and polished surface.

U.S. Pat. Nos. 6,299,755; 6,375,826; 6,599,415; and 6,679,980 disclosesystems for electropolishing stents. Although electropolishing is aneffective method for obtaining smooth surfaces on stents,electropolishing processes are very sensitive. Repeatable and evenpolishing is difficult to achieve. The uniformity of the polishedsurface depends on factors including on the uniformity of the currentdensity across the surfaces. The current density is particularly unevenat the contact points where the anode is in contact with the metalsurface. The high current density at the contact point result inparticularly aggressive polishing around the contact points. Inaddition, the metal contact member, which may be a clip, a wire, or aroller, can mask the current and cause a high point. These contactpoints result in contact marks on the polished products which areundesirable.

Accordingly, an electropolishing fixture which minimized current densitydifferences and substantially eliminates contact marks on the productsis needed. In addition, it would be desirable to provide overall evenelectropolishing.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, anelectropolishing device for polishing a substantially cylindricalmedical device comprises a plurality of anodes formed of a conductivematerial, the anodes in the form of rollers arranged to contact anexterior surface of the medical device, an insulated support structuresupporting the anodes, a drive mechanism for rotating the anodesperiodically during an electropolishing process to change a contactpoint between the anodes and the medical device, and a cathode connectedto the insulated support structure.

In accordance with an additional aspect of the present invention anelectropolishing device for polishing stents is comprised of a pluralityof rotatable anodes formed of a conductive material arranged to contactan exterior surface of the stents, a cathode. a drive mechanism forrotating the anodes periodically during an electropolishing process, anda current control device for turning the current off during rotation ofthe anodes and turning the current on to polish the stents when theanodes are not rotating.

In accordance with another aspect of the invention an electropolishingdevice for polishing stents is comprised of a plurality of rotatableanodes formed of a conductive material configured to support one or morestents, a drive mechanism for rotating the anodes periodically during anelectropolishing process, a tilting mechanism for tilting the anodesperiodically during the electropolishing process such that the stentsare non-horizontal during electropolishing to allow bubbles to escapefrom one end of the stent, and a cathode

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe preferred embodiments illustrated in the accompanying drawings, inwhich like elements bear like reference numerals, and wherein:

FIG. 1 is a perspective view of a portion of an electropolishing fixtureaccording to the present invention.

FIG. 2 is a perspective view of the electropolishing fixture of FIG. 1in a housing.

FIG. 3 is a side view of the electropolishing fixture housing of FIG. 2.

FIG. 4 is a back view of the electropolishing fixture of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a portion of a polishing fixture 10 for use inelectropolishing implantable medical devices, such as stents. Thepolishing fixture 10 is used to hold the medical devices when they areimmersed in an electrolyte solution in an electropolisher. FIG. 1illustrates two medical devices or stents 20 positioned in the fixture10. The stents 20 are laser cut from a metal tube to form expandable,substantially cylindrical devices.

The electropolisher into which the polishing fixture 10 and stents 20are immersed during electropolishing is not shown. Electropolishers arecommercially available and are used to polish and deburr small,precision machined, metallic items. The electropolisher equipment isgenerally constructed of stainless steel or from other non-corrodingalloys. The electrolyte solution is provided in a tank within theelectropolisher and the electrolyte solution temperature isautomatically controlled by a heater and cooling fan combination of theelectropolisher. The electropolisher also provides precision timing andelectrical control for the process which are connected to theelectropolishing fixture 10 and used to control the functions of thefixture.

Electrolyte solutions are commercially available from electropolishermanufacturers and are available for stainless-steel, cobalt chromiumalloys, nickel-chrome alloys, and other metals.

The polishing fixture 10 of FIG. 1 includes a plurality of anodes 30 anda cathode 40 formed of a conductive material. The polishing fixture 10is configured to rotate and tilt the stents and eliminate the need forstirring the electrolyte solution. Each of the anodes 30 include arotatable rod 32 and two anode rollers 34. A pair of anodes 30 eachhaving two rollers 34 support each stent 20 and provide electrical andmechanical contact at four points on each stent. A drive mechanism,which will be described below, rotates the anode rollers 34 and thus,rotates the stents periodically throughout the electropolishing processto change the contact points and limit the occurrence of contact markson the finished stent. The anodes 30 are arranged to contact onlyexterior surfaces of the stent 20 to eliminate contact marks on theinterior of the stent. Contact marks and other irregularities on theinterior surfaces of stents can possibly contribute to restenosis andthrombosis. The anode rollers 34 can be smooth or grooved. Although fouranode rollers 34 are illustrated supporting each stent, three rollers,or more than four rollers can also be used. However, limited numbers ofrollers are desirable to limit the contact marks and to improved theuniformity of flow of liquid around the stent.

In addition to the anodes 30, the stents 20 are retained within theelectropolishing fixture by movable idle rollers 50 formed of insulatingmaterial. The idle rollers 50 are mounted on idle roller arms 52 whichposition the idle rollers above the stents 20 to prevent the stents fromlosing electrical contact with the anodes 30. The idle rollers 50 can bemaintained in contact with the stents 20 during polishing by either theweight of the rollers and arms or by spring force of an associatedspring mechanism. As shown in FIG. 1, the idle rollers 50 can be movedout of the way for loading and unloading of the stents 20 by sliding asliding bar opening mechanism 54 in the direction of the arrow A torotate the idle roller arms 52 and idle roller support rods 56. Althoughone idle roller 50 per stent 20 is shown, additional idle rollers mayalso be provided.

The anode rods 32 and rods 56 on which the idle roller arms 52 rotateare rotatably mounted within an insulated support frame 60 of theelectropolishing fixture 10. The support frame 60 is designed to holdmultiple stents 20, such as 4-20 stents for simultaneouselectropolishing. Although each stent 20 is provided with multipleanodes 30 a single cathode 40 is provided which runs perpendicular tothe axes of the stents and beneath the stents to provide a consistentcurrent density to the multiple stents. The location and shape of thecathode will affect the uniformity of material removal. The rod cathoderather than a conventional screen or cylindrical cathode has been foundto provide more uniform polishing. The best configuration will depend onthe size and shape of the objects being polished.

The anode and cathode materials are selected depending on the materialof the stents 20 and the electrolyte solution. Electrode materials mayinclude titanium, nickel-titanium alloys, platinum, platinum-iridiumalloys, zirconium, and zirconium alloys. The support frame 60, housing80, and drive elements are made of acid resistant insulating polymermaterials, such as ultra high molecular weight polyethylene or Kynar.

The spacing between the anode rollers 34 is customized to accommodatethe particular medical device being polished. Many stent designs includea plurality of cylindrical members which provide radial support to avessel wall and which are interconnected by thinner and more flexiblebridging elements (not shown) which provide axial flexibility to thestent. A typical stent includes a repeating pattern of cylindricalelements and bridging elements. Since the bridging elements are thinnerthan the cylindrical elements of the stent they are less suitable forconducting larger polishing currents to the part. Therefore, the anodesshould be spaced so that both anodes will not be positioned on stentbridging elements. Thus, the anode rollers 34 should be positioned onthe rods 32 at a spacing which is different from the repeating patternof the stents.

The spacing between the anode rods 32 in each pair is dependant on thediameters of the stents 20 and the rollers 32. The diameter of the anoderollers 34 is at least 50% of the diameter of the stents 20, andpreferably at least equal to the diameter of the stents.

The anode rollers 34 are rotated with each roller in a pair rotating inthe same direction by a drive mechanism. The drive mechanism may includeany combination of drive elements, such as belts and gears. The rotationof the anode rollers 34 periodically throughout the electropolishingprocess moves the anode contact point on the stent and minimizes theoccurrence of contact marks on the polished stents 30. For a typicalabout 8 minute electropolishing process, the contact point is adjustedby rotation of the anode rollers sixteen times, or about every 30seconds. The rotation can be any amount which results in a differentcontact point at each rotation. For example, the stent can be rotatedabout 20 degrees, 340 degrees, or 380 degrees at each rotation. Thispattern assures maximum evenness of polishing in the radial direction.The polishing current is turned off or disconnected when the two anoderollers 34 are turning to prevent arcing that might mark the surface.Because the rate of polishing is proportional to the electric current, aseparate constant current power supply is used for each stent.

FIGS. 2 and 3 illustrate the polishing fixture 10 mounted in aninsulating housing 80. The polishing fixture 10 is rotatable in thehousing 80 about an axis 82 to allow the stents 20 to be tipped back andforth during electropolishing. Tipping the stents 20 will allow bubblesto escape from either end of the stent. The axis 82 about which thefixture 10 is rotated is substantially perpendicular to an axis of thestents 20 placed on the anodes 30. The tilting of the stents 20 duringelectropolishing improves the longitudinal uniformity of theelectropolishing process by varying the way in which bubbles createdduring polishing escape from an interior of the stent. These bubbles canescape from an end of the stent more easily when the stent is tilted.However, the bubbles can also increase turbulence at the end of thestent as they escape which can increase the polishing at this area.Therefore, the polishing fixture 10 is preferably tilted at least onceduring polishing to distribute the high polishing areas caused by thebubbles evenly to both ends of the stent. The fixture 10 can also betilted repeatedly during polishing. The degree of tilting can vary fromabout 3 degrees to about 45 degrees, and is preferably in the range ofabout 5 degrees to about 20 degrees. As shown in FIG. 3, the tilting ofthe polishing fixture 10 is provided by an actuator rod 84 which isdriven by any known drive mechanism. The polishing current is turned offduring tilting of the polishing fixture 10 to prevent arcing that mightmark the stent.

A motor 90, shown in FIG. 2, is provided on the polishing fixture 10 forrotating the rods 32 of the anodes 30. FIG. 4 shows the back side of thepolishing fixture 10 and the drive mechanism arrangement for rotatingthe anodes 30. Although the drive mechanism shown and described hereinincludes a combination of belts and gears, other drive systems can alsobe used. The drive mechanism shown includes a plurality of gears 92connected to one of each pair of anodes and a belt 94 connecting themotor 90 to the gears. Each of the pairs on anodes 30 is interconnectedby an anode drive belt 96 on the front side of the polishing fixture 10as shown in FIGS. 1 and 2. Thus, the rotation of the motor 90 istransmitted by the belt 94, the gears 92, and the anode belts 96 to eachof the anode rollers 34 to rotate the stents 20.

FIG. 4 also illustrates one example of a electrical contact system forconnection to each of the anodes 30. A spring loaded anode clip 100connected to an electrical lead 102 is pressed into contact with eachpair of anode rods 32 to provide electrical contact with the anodes. Thespring loaded clip allows the anode rods 32 to rotate while maintainingelectrical contact with the rods. Each of the anode pairs isindependently controlled by the electropolisher electrical controlsystem.

While the invention has been described in detail with reference to thepreferred embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made and equivalentsemployed, without departing from the present invention.

1. An electropolishing device for polishing a substantially cylindricalmedical device, the electropolishing device comprising: a plurality ofanodes formed of a conductive material, the anodes in the form ofrollers arranged to contact an exterior surface of the medical device;an insulated support structure supporting the anodes; a drive mechanismfor rotating the anodes periodically during an electropolishing processto change a contact point between the anodes and the medical device; anda cathode connected to the insulated support structure.
 2. Theelectropolishing device of claim 1, wherein the plurality of anodes arearranged for contact only with the exterior surface of the medicaldevice.
 3. The electropolishing device of claim 1, wherein the drivemechanism rotates the plurality of anodes at least 4 times during theelectropolishing process.
 4. The electropolishing device of claim 1,wherein the drive mechanism rotates the plurality of anodes periodicallyand the periodic rotation of the anodes results in rotation of themedical device about one revolution.
 5. The electropolishing device ofclaim 1, wherein the plurality of anodes each include a rod ofconducting metal with at least two rollers fixed to the rod.
 6. Theelectropolishing device of claim 5, wherein the rods are arranged on theinsulating support structure as a plurality of parallel pairs of rods,with each pair of anodes supporting a medical device thereon.
 7. Theelectropolishing device of claim 1, wherein the insulating supportstructure is movable into and out of an electrolytic solution.
 8. Theelectropolishing device of claim 1, wherein the insulating supportstructure is positioned in a housing and is rotatable in the housingabout an axis substantially perpendicular to an axis of a medical deviceplaced on the anodes to allow the medical devices to be tipped duringthe electropolishing process.
 9. The electropolishing device of claim 1,wherein the plurality of anodes are formed of titanium.
 10. Theelectropolishing device of claim 1, wherein the cathode is a wire rodpositioned in the insulating support and running substantiallyperpendicular to the axis of a medical device placed on the anodes. 11.The electropolishing device of claim 1, wherein the cathode is azirconium wire.
 12. The electropolishing device of claim 1, wherein theplurality of anodes provide four contact points on an exterior of themedical device.
 13. The electropolishing device of claim 1, furthercomprising an insulating idle roller arranged to maintain the medicaldevice in contact with the anodes during the electropolishing process.14. The electropolishing device of claim 13, wherein the idle rollerprovides one contact point with the medical device.
 15. Theelectropolishing device of claim 1, further comprising an insulatingidle roller arranged to maintain the medical device in contact with theanodes during the electropolishing process.
 16. An electropolishingdevice for polishing stents, the device comprising: a plurality ofrotatable anodes formed of a conductive material arranged to contact anexterior surface of the stents; and a cathode arranged to be positionedoutside of the stents, wherein no electrodes are positioned inside thestents.
 17. The electropolishing device of claim 16, further comprisinga drive mechanism for rotating the plurality of anodes.
 18. Theelectropolishing device of claim 16, wherein the insulating supportstructure is positioned in a housing and is rotatable in the housingabout an axis substantially perpendicular to an axis of a medical deviceplaced on the anodes to allow the medical devices to be tipped duringthe electropolishing process.
 19. The electropolishing device of claim16, wherein the cathode is a wire rod positioned in the insulatingsupport and running substantially perpendicular to the axis of a medicaldevice placed on the anodes.
 20. The electropolishing device of claim16, wherein the plurality of anodes provide four contact points on anexterior of the medical device.
 21. An electropolishing device forpolishing stents, the device comprising: a plurality of rotatable anodesformed of a conductive material arranged to contact an exterior surfaceof the stents; a cathode; a drive mechanism for rotating the anodesperiodically during an electropolishing process; and a current controldevice for turning the current off during rotation of the anodes andturning the current on to polish the stents when the anodes are notrotating.
 22. The electropolishing device of claim 21, wherein theanodes are mounted on a support structure which is movable into and outof an electrolytic solution.
 23. The electropolishing device of claim21, wherein the plurality of anodes are formed of titanium.
 24. Theelectropolishing device of claim 21, wherein the plurality of anodesprovide four contact points on each stent.
 25. The electropolishingdevice of claim 21, further comprising an insulated idle roller which ismovable into contact with the stents to maintain the stents in contactwith the anodes.
 26. The electropolishing device of claim 21, whereinthe electropolishing device polishes stents without stirring of anelectrolytic solution.
 27. The electropolishing device of claim 21,wherein the cathode is a fixed cathode which is not in the shape of ascreen, mesh, or cylinder.
 28. An electropolishing device for polishingstents, the device comprising: a plurality of rotatable anodes formed ofa conductive material configured to support one or more stents; a drivemechanism for rotating the anodes periodically during anelectropolishing process; a tilting mechanism for tilting the anodesperiodically during the electropolishing process such that the stentsare non-horizontal during electropolishing to allow bubbles to escapefrom one end of the stent; and a cathode.
 29. The electropolishingdevice of claim 28, further comprising a current control device forturning the current off during tilting of the anodes.
 30. Theelectropolishing device of claim 29, wherein the tilting mechanism tiltsthe anodes at least once during the electropolishing process to allowbubbles to escape from each end of the stent.